The present invention relates to a monopulse phased array system, in particular for a volumetric array antenna whose antenna elements are spatially arranged in three dimensions and which is commonly referred to as crow""s nest antenna (CNA). In a special embodiment, the antenna elements are arranged according to a three-dimensional surface, in which case the antenna is designated as xe2x80x98conformal arrayxe2x80x99 antenna. However, the invention can also be utilized in known linear or planar phased array systems.
More concretely, the invention relates to a monopulse phased array system comprising a number of antenna elements, each of which is connected to a T/R (transmitter/receiver) module, which is under the control of a beam steering computer (BSC), to which T/R module a transmitting signal is fed for forming a transmitting beam, and from which, upon reception of reflected signals, a sum signal, a first angle difference signal, in particular an elevation difference signal, and a second difference signal, in particular an azimuth difference signal, are taken, in which phased array system, further, for each of the three signals received via one antenna element, at least one combination unit is present to derive therefrom a total sum signal (xcexa3), a total first angle difference signal (xcex94E) and a total second angle difference signal (xcex94A), from which last-mentioned signals regulation signals (xcex94E/xcexa3) and (xcex94A/xcexa3) for resteering the transmitting beam generated under the control of the beam steering computer can be obtained. Such a monopuls phased array system is known from U.S. Pat. No. 5,173,702.
In general, tracking a target with a monopulse tracking device requires the simultaneous generation of two types of receiving patterns which are used for determining the angular position of the target relative to the main axis of the monopulse tracking beam. The first receiving pattern is a sum pattern (xcexa3-pattern), which consists of one single main lobe and a number of much weaker side lobes. The main lobe of the sum pattern points in the direction of the target from which the reflected signals originate and is used to normalize angle information obtained with the aid of the second receiving pattern. The second receiving pattern is a difference pattern (xcex94E or xcex94A pattern), which consists of two nominally equal-strength main lobes, and a number of much weaker side lobes. The axis of the sum pattern and that of the difference patterns are identical; the patterns are directed to the target along the same line. The ratio of the measured signal strength in the difference patterns, that is, the strength of the measured difference signals, to the measured signal strength in the sum signal, that is, the strength of the measured sum signal, is used to determine the angular position of the target. In practice, it is conventional to generate one sum pattern and two difference patterns to enable independent tracking of a target in azimuth and elevation.
To be able to apply the monopulse principle in a CNA, as described in H. Wilden c.s., The crow""s nest antenna-experimental results, IEEE International Radar Conference, Arlington, May 7, 1990, p.280-285, it is known to divide the space in which the antenna elements are arranged into eight sectors (octants). The signals received in the respective sectors, when added together, yield the sum signal (xcexa3). At the same time, different combinations of selected sectors are used to derive three difference signals, which are related to the internal coordinate system of the CNA: for instance, the difference of the signals received in four upper and four lower octants, in four left and four right octants, and in four front and four rear octants. These difference signals are further processed to derive from them an elevation difference signal (xcex94E) and an azimuth difference signal (xcex94A) for resteering the emitted beam to a target.
Various disadvantages are inherent to this known elaboration and application of the monopulse principle. A complicated monopulse RF hardware is present to obtain the three different signals for the CNA coordinate system. Extra signal processing means must be present to derive from these difference signals the elevation and azimuth difference signal. Placing the antenna elements in CNA configuration, which in itself is already complicated due to the large number of antenna elements, is rendered more difficult by the necessity of defining within the system fixed octants that are independent of each other as much as possible, i.e., that the signals received within them influence each other as little as possible. Further, an imbalance arises between the two difference patterns, which imbalance is greater according as a target is located at a relatively greater angle with respect to the octant boundaries, that is, at a greater angle with respect to the internal coordinate axes of the CNA, so that the accuracy with which a target can be tracked by means of the monopulse device diminishes.
The object of the invention is to provide a monopulse phased array system in which the above-mentioned disadvantages are at least substantially avoided.
To achieve this object, the monopulse phased array system such as it is described in the preamble is characterized in that the space in which the antenna elements are arranged is divided into quadrants which are defined relative to the main axis of the transmitting beam, while the axes defined for determining the first and second angle difference signal are directed perpendicularly to the main axis of the transmitting beam; that each of the T/R modules, both for determining the first angle difference signal and for determining the second angle difference signal, is provided with a 180xc2x0 phase shifter; and that when the transmitting beam after rotation has been directed to a target, in the T/R modules of the antenna elements which are located in sectors whose sign of the axis direction perpendicular to the axis direction of the transmitting beam has changed through the rotation, a phase shift of 180xc2x0 is effected in the first and/or in the second angle difference signal. This feature basically means that the regulation curve of the monopulse tracking system becomes dependent on the direction of the transmitting beam.
What is achieved through these measures is that the illumination of the two virtual halves on opposite sides of the plane through the axis of the main beam and the respective axis perpendicular thereto yields difference patterns that are virtually completely in balance with each other. Further, the first and second angle difference signal are obtained without conversion of a fixed division into octants to target coordinates. Complicated and relatively expensive monopulse RF hardware is redundant; instead, for each T/R module, a signal inversion corresponding to a 180xc2x0 phase shift can suffice. More freedom in the spatial arrangement of the antenna elements is obtained. Moreover, the monopulse phased array system is now no longer limited to tracking a target in azimuth and elevation: the target can also be tracked at different angles. Thus, a specific tracking direction and hence a specific difference pattern can be oriented along the track of the target, and tracking the target in the respective direction perpendicular thereto affords the possibility of determining the transverse component of the target velocity, which, along with the Doppler shift in the reflected signal, yields all of the components of the target velocity. As a result, the accuracy and the run-in time of target tracking filters in a radar processor will be improved.
In the monopulse phased array system according to the invention, it is sufficient that for each of the three signals received via one antenna element, only one combination unit is present. In it, all signals are added up in the proper phase, that is, shifted 180xc2x0 in phase or not. To enable this, each of the T/R modules is provided with switching means for feeding the first and/or the second angle difference signal, shifted 180xc2x0 or not, to the combination unit in question.
The invention will now be further explained on the basis of the accompanying drawing, wherein:
FIG. 1 schematically shows an arrangement of antenna elements and the manner in which the T/R modules are combined according to the prior art;
FIG. 2 shows a diagram on the basis of which the imbalance in the difference patterns in a monopulse phased array system according to the prior art will be explained;
FIG. 3 schematically shows an arrangement of antenna elements and the manner in which the T/R modules are combined according to the invention;
FIG. 4 shows a diagram on the basis of which it will be explained how the imbalance in the difference patterns is prevented in a monopulse phased array system according to the invention; and
FIG. 5 shows a T/R module according to the invention.